Fudala, Nicholas J. and Michael D. Fontaine
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In July 2008, a variable speed limit (VSL) system was implemented on a 7.5 mile section of I-495 (the Capital Beltway) in Virginia between the Springfield Interchange and the Woodrow Wilson Bridge. A study was conducted to examine performance and evaluate system impacts, and a literature search was performed to identify measures of effectiveness (MOEs) and gather information on VSL system and algorithm design.
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High-volume urban work zones tend to be prone to congestion and safety problems, and Variable speed limit (VSL) systems may be one way to ameliorate these issues. VSL signs were installed at a high-volume, congested, urban work zone located on I-495 (the Capital Beltway) between the Springfield Interchange in Springfield, Virginia, and the Virginia-Maryland state line on the Woodrow Wilson Memorial Bridge (WWB). The Virginia Department of Transportation (VDOT) sponsored an evaluation of the effectiveness of these VSL signs deployed in a work zone environment.

The VSL signs were activated in late July 2008, but initial evaluations of the system showed inconclusive effects. Changing site conditions in a work zone made a direct before-and-after evaluation of the system deployed in the field problematic, and some problems with the control algorithm were also noted. Given the difficulties in evaluating the system deployed in the field, a calibrated simulation of the site was constructed to assess the effects of the VSL system on traffic operations and safety surrogate measures.

Work zone lane closures were simulated by removing lanes that were closed because of construction. VSL signs were simulated at the same locations as in the field deployment. As such, the sign locations may not have been optimal for the specific lane closure simulated, but they were chosen to allow for consistency between the field and simulation environments.

The results indicated that the VSL could create substantial improvements in traffic operations provided the demand did not exceed capacity by too large a margin. Further, a cost/benefit analysis indicated that VSL signs may be most appropriate for long-term applications
Firoz Kabir
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Virginia DOT
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Work Zone Variable Speed Limit Systems: Effectiveness and System Design Issues
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